Water slowly disappering from reserve

What is your question?

the water and coolant is slowly disappearing from the reserve tank. 4 days ago the reserve tank was full and now it is almost empty. The temperature guage never goes up, but it dose seem that th engine is getting really hot. The major hose from the radiator to the engine was recently changed because it was bloated. How can I tell if the engine is getting hot or if a head gasket might blow. The reserve gets lower and lower.